Mid-Infrared Quantum Cascade Lasers

Home , Heterojunction, Potential well, Quantum cascade laser

MID-INFRARED QUANTUM CASCADE LASERS

S.O. Edeagu Nanoelectronics Research Group, Department of Electronic Engineering, University of Nigeria, Nsukka, Nigeria. Email: [email protected]

Abstract Quantum cascade lasers (QCL) based on intersubband transitions operating at room temperature in the mid-infrared or ‘molecular ﬁngerprint’ spectral region (3.4–17 µm) have been found useful for several applications including environmental sensing, pollution monitoring, and medical applications. In this tutorial review we present an introductory overview of the design and development of mid-infrared quantum cascade lasers and their applications.

Keywords: laser, intersubband transitions, quantum cascade, mid-infrared, quantum well

1. Introduction known as the Bernard-Duraffourg condition. In late 1962, the first reports of the operation of semicon- Semiconductor lasers have become ubiquitous de- ductor injection lasers were published. The four inde- vices in today’s modern high-tech society. They can pendent groups were Hall and co-workers at the GE be found in familiar devices such as bar-code scanners Research Lab, New York [7], Nathan and co-workers used in supermarkets, laser printers in offices, compact at the IBM T.J. Watson Research Center, New York disc and digital versatile disc (DVD) players at home [8], Holonyak and co-workers at the GE Advanced and laser pointers used during presentations. They Semiconductor Lab, New York [9] and Quist and co- are widely used in industry for example, cutting and workers at Lincoln Lab, MIT [10]. These lasers were welding metals and other materials, in medicine for achieved using GaAs p-n junctions. surgery, in optical communications, in optical metrol- In 1994, a fundamentally different kind of laser ogy and in scientific research. diode known as the quantum cascade laser (QCL) The laser is theoretically based on Einstein’s clas- emerged. During the last decade significant improve- sic paper of 1917, in which he introduced the con- ments made in quantum cascade lasers has led to cept of stimulated or induced emission of radiation its gradual emergence as the mid-infrared source of [1]. The first demonstration of stimulated emission choice. The quantum cascade laser’s unique proper- took place in 1954 when Townes and co-workers used ties, such as its small size, high output power, tunabil- a beam of ammonia (NH3) gas, in a microwave de- ity of the emission wavelength, and ability to work in vice called the maser (microwave amplification by pulsed and continuous wave mode at room tempera- stimulated emission radiation) [2]. It generated a co- ture, make it ideal for the remote sensing of environ- herent source of microwaves at 23.87 GHz. In 1958, mental gases and pollutants. Schawlow and Townes proposed a theory extending stimulated emission to optical and infrared frequen- cies [4]. Two years later, the optical equivalent of the 2. Basic Concepts maser - the laser (light amplification by stimulated emission radiation) - was demonstrated by Maiman Conventional semiconductor lasers operate via ra- [5]. He used a three-level ruby laser emitting light of diative recombination of an electron in the conduc- wavelength 693 nm. tion band with a hole in the valence band (band-band The first theoretical understanding of the re- transition or inter-band transition). The recombina- quirements for the realization of stimulated emis- tion generates a photon. Fig. 1 shows a schematic sion in a semiconductor was developed by Bernard diagram of an interband transition. Due to the fact and Duraffourg of the Centre National d’Etudes des that two types of carriers, electron and holes, are in- Télécommunications (CNET), France and published volved, semiconductor lasers are also known as bipolar in 1961 [6]. The equations derived by them became lasers. In contrast, the quantum cascade laser, makes

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Figure 1: Interband optical transitions between conﬁned states in a quantum well [13].

Figure 2: (a) Intersubband absorption in an n-type quantum use of only a single carrier, the electron. The radia- well. (b) Intersubband emission following electron injection to tive recombination takes place within a single band a confined upper level in the conduction band [13]. between two bound states of a potential well (intra- band transition or intersubband transition), as illus- trated in Fig. 2. Through so-called electron recycling, material system lattice-matched to InP [14]. Over a single electron generates multiple photons. the years other material systems have been used to Quantum cascade lasers (QCLs) [14] are semicon- demonstrate QCLs. In 1998, Sirtori and cowork- ductor lasers which rely on inter-subband transitions ers used the GaAs/AlGaAs on GaAs material sys- in a multiple quantum-well heterostructure. The tran- tem [20]. Others include InAs/AlSb [21] [22] and sitions typically occur in the mid- and far-infrared InGaAs/AlAsSb [23]. These designed quantum well spectral region. QCLs are based on two fundamental structures are grown by using thin-film crystal growth phenomena of quantum mechanics, namely tunneling techniques such as molecular beam epitaxy (MBE) or and quantum confinement. metal-organic chemical vapor deposition (MOCVD) The original idea for creating the quantum cascade [18]. laser was first postulated in 1970 by two theoreticians, The design of a QCL consists of one basic struc- Kazarinov and Suris of the Ioffe Institute in Leningrad ture called a period or stage, repeated multiple times (now St. Petersburg) [15]. They proposed that one (typically 25 to 75). Each period consists of an ac- could obtain light amplification based on intersubband tive region, where the optical transition takes place, transitions in quantum wells electrically pumped by and an relaxation/injector region, where the carriers resonant tunneling. Their proposal came shortly after can relax after having completed the optical transi- a seminal paper on the superlattice concept by Esaki tion. The active region is undoped and contains the and Tsu of the IBM Research Labs [16]. quantum wells and barrier regions that support the However due to the lack of appropriate technology electronic states while the relaxation/injector region in the 1970s, it took more than twenty years before the is n-doped. first quantum cascade laser was invented and demon- For the QCL invented by Faist et al. in 1994, strated by the Bell Labs groups of Capasso and Cho the laser structure consisted of 25 stages with the in 1994 [14][17]. This achievement was largely made Al0.48In0.52As-Ga0.47In0.53-As heterojunction mate- possible by the emergence of molecular beam epitaxy rial system lattice matched to InP [14]. Each of the (MBE) as a practical crystal growth technology for periods consisted of four AlInAs barriers forming three quantum well heterostructures, largely pioneered by GaInAs quantum wells and a digitally graded AlIn- Cho at Bell Laboratories [18]. GaAs region that is doped. Quantum cascade lasers, also known as unipolar Under an applied bias (an applied electric field of lasers, are designed by means of band-gap engineer- 105 V/cm) the band edges of the quantum wells are ing [19]. The art of band-gap engineering enables one slightly tilted forming a staircase structure. The carri- to engineer new materials by tuning their band gap. ers (electrons) are injected by resonant tunneling from The first QCL was constructed from a concatenated the ground state of one active region to the upper laser series of quantum wells, using the GaAs/AlxIn1−xAs level of the active region in the neighboring period.

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Figure 3: Schematic diagram of the conduction band for two periods of a quantum cascade laser with a quantum well active region [14]. The laser transition is indicated by the wavy arrows, Figure 4: Schematic diagram of a superlattice QCL with a and the electron flow by the straight arrows. miniband-to-miniband radiative transition [26]. The laser transition is indicated by the wavy arrows, and the electron flow by the straight arrows. The electron drops in energy level emitting a photon in the process. As the electron moves from one stage laser frequency is thus determined by the height of the to another, it emits multiple photons in a cascade pro- minigap. The extremely short electron lifetime (decay cess, as the electron remains inside the conduction occurs rapidly) from the lower laser-level miniband band throughout its traversal of the active region. makes this structure suitable for longer wavelengths (λ ≥ 10µm) [27–30]. 3. Active Region Designs 3.3. Bound-to-continuum active region There are two major designs for the active region of quantum cascade lasers: quantum-well active region In a bound-to-continuum design [31], transitions oc- and superlattice active region [11]. A third design, cur between a discrete upper state and a lower super- the so-called bound-to-continuum scheme, combines lattice miniband. This design [see Fig. 5] combines the features of the quantum-well QCL and the super- the efficient electron injection into the upper laser level lattice QCL. Various other designs for the QCL active of the quantum-well design with the fast depopula- region have been developed [24], which is far beyond tion/electron extraction efficiency of the lower laser the scope of this article. level of the superlattice design, thereby reducing the threshold and increasing the power. 3.1. Quantum-well active region The laser action involves transitions from a discrete The quantum-well active region is based on the upper state to a superlattice miniband. This design three-level laser system. The active region consists of combines the efficient electron injection into the up- three asymmetric coupled quantum wells (3QW), as per laser level of quantum-well QCLs with the fast shown in Fig. 3, and the laser transition takes place depopulation of the lower laser level of superlattice between the upper laser state (level 3) and the lower QCLs, thereby reducing the threshold and increasing laser state (level 2) of the coupled system. Popula- the power. tion inversion is only possible when the lifetime of the 3→2 transition (τ32 ≈ 1 ps) is longer than the lifetime 4. Quantum Cascade Laser Configurations of level 2 (τ2 ≈ τ21 ≈ 0.1 ps). Since τ32 τ2, laser action is possible. 4.1. Fabry-Pérot quantum cascade lasers 3.2. Superlattice active region This is the simplest of the quantum cascade lasers. The superlattice design [see Fig. 4.] was first pro- An optical waveguide is first fabricated out of the posed by Scamarcio in 1997 [26]. In this design, stim- quantum cascade material to form the gain medium. ulated emission takes place between superlattice en- The ends of the crystalline semiconductor device are ergy bands (mini-bands) separated by an energy gap then cleaved (i.e. broken) to form two parallel mirrors called a minigap. Electrons are injected near the bot- on either end of the waveguide, thus forming a Fabry- tom of an upper miniband which then makes an opti- Pérotresonator. The optical reflection (feedback) is cal transition to the top of the lower miniband. The provided by the borders of the resonator (facets). In

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Figure 5: Schematic diagram of a bound-to-continuum QCL Figure 6: Schematic of a distributed feedback quantum cas- [31]. The laser transition is indicated by the wavy arrow. cade laser [33]. many cases, the high refractive index difference be- introduced in the early 1970s by Kogelnik and Shank tween the semiconductor and the air is already suf- [36]. ficient, providing an optical reflection coefficient of about 30%. By additional facet coatings this coef- 4.3. External cavity quantum cascade lasers ficient can be tailored continuously between 0% (antireflection) and 100% (perfect reflection). Conventional quantum cascade lasers are at most Fabry-Pérotquantum cascade lasers are capable of only narrowly tunable. The DFB QCL does not pro- producing high powers [32], but are typically multi- vide a broadly tunable laser for the mid-IR. The range mode at higher operating currents. The wavelength of wavelength tuning of the emitted laser radiation is can be changed chiefly by changing the temperature limited by the limited tuning range of the distributed of the QCL. feedback structures. Typically the thermal tuning range of DFB QCLs is achieved by varying either the 4.2. Distributed feedback quantum cascade temperature of the chip or the laser injection current. lasers By varying the injection current and/or temperature, QCLs have been touted as a source for numerous the effective refractive index is modified which in turn commercial applications, one of which is gas sensing changes the optical path length of the cavity and thus applications, e.g. to detect very small concentrations the emission wavelength. One of the disadvantages of pollutants in air. These applications usually re- of such a thermal tuning is that it affects the effective quire single mode operation with good side-mode sup- gain of the QCL, which consequently causes a decrease pression ratio (SMSR)1 which can be controlled and of the output laser power with increasing temperature tuned over a certain wavelength range. This is usually of the QCL chip. To overcome these drawbacks, the achieved by introducing a distributed feedback (DFB) external cavity quantum cascade laser system was de- structure into the QCL active region [see Fig. 6]. veloped [37–39]. Distributed feedback QCLs require a periodic mod- In the external cavity configuration, the QCL is cou- ulation (i.e., a grating) of a material parameter (e.g., pled to an external cavity that incorporates a diffrac- the refractive index or loss coefficient) to be integrated tion grating as a wavelength-selective element, which into the device. Repeated scattering from the first- then provides frequency-selective optical feedback to order grating favors one wavelength (the Bragg wave- the QCL via its antireflection-coated output facet. length) and it is the grating period rather than the This concept of frequency selective feedback allows the peak position of the gain spectrum that determines laser to achieve narrow linewidth and remarkable tun- the single-mode emission wavelength. ability. Typically, such feedback is obtained through DFB QCLs were first demonstrated in 1996 [34,35] diffraction grating in either the Littrow or Littman- and is based on the concept of distributed feedback Metcalf configuration. The most popular arrangement of an external cavity of a QCL, shown in Fig. 7, is based on the grating 1 SMSR = 10 log10(P1/P2) where P1 and P2 ≤ P1 denote feedback in the Littrow configuration. The QCL chip the output power of the two strongest modes in the optical spectrum. The SMSR for DFB QCLs readily reaches 30 dB, is mounted on a thermoelectric (TE) cooler and has which qualifies them as ‘single-mode’ lasers. its front facet anti-reflection (AR) coated. In such

Nigerian Journal of Technology Vol. 31, No. 3, November 2012. Mid-Infrared Quantum Cascade Lasers 231 6. Conclusion

In this paper, we have discussed the design and development of quantum cascade lasers in the mid- infrared spectral range which have unique features that are well adapted for applications ranging from chemical sensing and detection, remote sensing to free- space optical communications.

References

1. Einstein, A. Zur Quantentheorie der Strahlung (On the Quantum Theory of Radiation). Phys. Z., Vol. 18, 1917, pp. 121-128. 2. Gordon, J. P., Zeiger, H. J. and Townes, C. H. Molec- ular Microwave Oscillator and New Hyperfine Struc- Figure 7: External-cavity setup (Littrow configuration). ture in the Microwave Spectrum of NH3. Phys. Rev., Vol. 95, Issue 1, 1954, pp. 282-284. 3. Gordon, J. P., Zeiger, H. J. and Townes, C. H. The configuration, the grating reflects back to the QCL Maser - New Type of Microwave Amplifier, Frequency the radiation in non-zero diffraction order (first-order Standard, and Spectrometer. Phys. Rev., Vol. 99, feedback). The laser output coupling is performed Issue 4, 1955, pp. 1264-1274. through zero diffraction order of the grating. 4. Schawlow, A. L. and Townes, C. H. Infrared and Op- tical Masers. Phys. Rev., Vol. 112, Issue 6, 1958, pp. 5. Applications 1940-1949. 5. Maiman, T. H. Stimulated Optical Radiation in The mid-infrared (MIR) spectral region, is some- Ruby. Nature, Vol. 187, Issue 4736, 1960, pp. 493- times called the fingerprint region of the electromag- 494. netic spectrum, because most molecules have their 6. Bernard, M. G. A. and Duraffourg, G. Laser Condi- fundamental vibrational-rotational absorption bands tions in Semiconductors. Phys. Stat. Sol., Vol. 1, in this range. The MIR absorption spectrum is very Issue 7, 1961, pp. 699-703. specific to the structure of a particular molecule, al- 7. Hall, R. N., Fenner, G. E., Kingsley, J. D., Soltys, T. lowing highly selective detection. In addition, since J. and Carlson, R. O. Coherent light emission from these absorption lines are very strong (several orders GaAs junctions. Phys. Rev. Lett., Vol. 9, Issue 9, of magnitude stronger than the overtone and combi- 1962, pp. 366-368. nation bands in the NIR), concentrations in the parts- 8. Nathan, M. I., Dumke, W. P., Burns, G., Dill, Jr., F. per-billion (ppb) to parts-per-trillion (ppt) ranges can H. and Lasher, G. Stimulated emission of radiation be detected using relatively compact laser-based sen- from GaAs p-n junctions. Appl. Phys. Lett., Vol. 1, sors. Fast, sensitive, and selective chemical sensors Issue 3, 1962, pp. 62-64. are needed in numerous applications. In industrial 9. Holonyak, Jr. N. and Bevacqua, S. F. Coherent process control they are used for detection of con- (visible) light emission from Ga(As1−xPx) junctions. tamination in semiconductor fabrication lines and for Appl. Phys. Lett., Vol. 1, Issue 4, 1962, pp. 82-83. plasma monitoring, in law enforcement for drug and 10. Quist, T. M., Rediker, R. H., Keyes, R. J., Krag, explosive detection, in automotive industry for engine W. E., Lax, B., McWhorter, A. L. and Zeiger, H. J. exhaust analysis, in environmental science for pollu- Semiconductor maser of GaAs. Appl. Phys. Lett., tion monitoring of important gas species (CO, CO2, Vol. 1, Issue 4, 1962, pp. 91-92. CH4 and CH2O), in medical diagnostics for exhaled 11. Saleh, B. E. A. and Teich, M. C. Fundamentals of breath analysis (e.g. NO, CO, CO2, NH3 and CS2), Photonics. Second Edition, John Wiley and Sons, and in homeland security for detection of chemical New Jersey, 2007. warfare agents. 12. Tittel, F. K., Richter, D. and Fried, A. Solid-State Another interesting feature of the MIR are the at- Mid-Infrared Laser Sources, Vol. 89 of Topics Appl. mospheric transmission windows between 3 - 5 µm Phys. ch. Mid-Infrared Laser Applications in Spec- and 8 - 12 µm which enable free-space optical commu- troscopy. Sorokina, I. T. and Vodopyanov, K. L. nications, remote sensing, and thermal imaging. High (Eds.) Springer, Berlin, 2003, pp. 445-516. power lasers in the 3 - 5 µm range will also enable the 13. Fox, M. Springer Handbook of Electronic and Pho- development of infrared counter-measures for home- tonic Materials. Springer, New York, 2006, pp. 1021- land security. 1040.

Nigerian Journal of Technology Vol. 31, No. 3, November 2012. 232 S.O. EDEAGU

14. Faist, J., Capasso, F., Sivco, D. L., Sirtori, C., 28. Tredicucci, A., Gmachl, C., Capasso, F., Hutchin- Hutchinson, A. L. and Cho, A. Y. Quantum Cascade son, A. L., Sivco, D. L. and Cho, A. Y. Single-mode Laser. Science, Vol. 264, Number 5158, 1994, pp. surface-plasmon laser. Appl. Phys. Lett., Vol. 76, 553-556. Issue 16, 2000, pp. 2164-2166. 15. Kazarinov, R. F. and Suris, R. A. Possibility of ampli- 29. Tredicucci, A., Gmachl, C., Wanke, M. C., Capasso, ﬁcation of electromagnetic waves in a semiconductor F., Hutchinson, A. L., Sivco, D. L., Chu, S. G. and with a superlattice. Sov. Phys. Semicond. (trans- Cho, A. Y. Surface plasmon quantum cascade lasers lated from Fizika i Tekhnika Poluprovodnikov, Vol. at λ ∼ 19µm. Appl. Phys. Lett., Vol. 77, Issue 15, 5, pp. 797-800, 1971), Vol. 5, 1971, pp. 707-709. 2000, pp. 2286-2288. 16. Esaki, L. and Tsu, R. Superlattice and Negative Dif- 30. Colombelli, R., Capasso, F., Gmachl, C., Hutchinson, ferential Conductivity in Semiconductors. IBM J. A. L., Sivco, D. L., Tredicucci, A., Wanke, M. C., Res. Dev., Vol. 14, 1970, pp. 61-65. Sergent, A. M. and Cho, A. Y. Far-infrared surface- plasmon quantum cascade lasers at 21.5 µm and 24 17. Capasso, F., Cho, A. Y., Faist, J., Hutchinson, A. µm wavelengths. Appl. Phys. Lett., Vol. 78, Issue L., Luryi, S., Sirtori, C. and Sivco, D. L. Unipolar 18, 2001, pp. 2620-2622. semiconductor laser. U.S. Patent 5 457 709, 1995. 31. Faist, J., Beck, M., Aellen, T. and Gini, E. Quantum- 18. Cho, A. Y. and Arthur, J. R. Molecular Beam Epi- cascade lasers based on a bound-to-continuum tran- taxy. Progress in Solid-State Chemistry, Vol. 10, Is- sition. Appl. Phys. Lett., Vol. 78, Issue 2, 2001, pp. sue 3, 1994, pp. 157-191. 147-149. 19. Capasso, F. Band-Gap Engineering: From Physics 32. Slivken, S., Evans, A., David, J. and Razeghi, and Materials to New Semiconductor Devices. Sci- M. High-average-power, high-duty-cycle (λ ∼ 6µm) ence, Vol. 235, Number 4785, 1987, pp. 172-176. quantum cascade lasers. Appl. Phys. Lett., Vol. 81, 20. Sirtori, C., Kruck, P., Barbieri, S., Collot, P., Issue 23, 2002, pp. 4321-4323. Nagle, J., Beck, M., Faist, J. and Oesterle, U. 33. Faist, J., Capasso, F., Sirtori, C., Sivco, D. L., Bail- GaAs/AlxGa1−xAs quantum cascade lasers. Appl. largeon, J. N., Hutchinson, A. L., Chu, S.-N. G. and Phys. Lett., Vol. 73, Issue 24, 1998, pp. 3486-3488. Cho, A. Y. High power mid-infrared (λ ∼ 5µm) quantum cascade lasers operating above room tempera- 21. Ohtani, K. and Ohno, H. InAs/AlSb quantum cas- ture. Appl. Phys. Lett., Vol. 68, Issue 26, 1996, pp. cade lasers operating at 10 µm. Appl. Phys. Lett., 3680-3682. Vol. 82, Issue 7, 2003, pp. 1003-1005. 34. Faist, J., Gmachl, C., Capasso, F., Sirtori, C., Sivco, 22. Teissier, R., Barate, D., Vicet, A., Alibert, C., Bara- D. L., Baillargeon, J. N. and Cho, A. Y. Distributed nov, A. N., Marcadet, X., Renard, C., Garcia, M., feedback quantum cascade lasers. Appl. Phys. Lett., Sirtori, C., Revin, D. and Cockburn, J. Room tem- Vol. 70, Issue 20, 1997, pp. 2670-2672. perature operation of InAs/AlSb quantum cascade lasers. Appl. Phys. Lett., Vol. 85, Issue 2, 2004, 35. Gmachl, C., Faist, J., Baillargeon, J. N., Capasso, pp. 167-169. F., Sirtori, C., Sivco, D. L., Chu, S. N. G. and Cho, A. Y. Complex-coupled quantum cascade distributed- 23. Revin, D. G., Wilson, L. R., Zibik, E. A., Green, feedback laser. IEEE Photon. Technol. Lett., Vol. 9, R. P., Cockburn, J. W., Steer, M. J., Airey, R. J. Issue 8, 1997, pp. 1090-1092. and Hopkinson, M. InGaAs/AlAsSb quantum cascade lasers. Appl. Phys. Lett., Vol. 85, Issue 18, 36. Kogelnik, H. and Shank, C. V. Coupled-Wave Theory 2004, pp. 3992-3994. of Distributed Feedback Lasers. J. Appl. Phys., Vol. 43, Issue 5, 1972, pp. 2327-2335. 24. Gmachl, C., Capasso, F., Sivco, D. L. and Cho, A. 37. Luo, G. P., Peng, C., Le, H. Q., Pei, S. S., Hwang, Y. Recent progress in quantum cascade lasers and W.-Y., Ishaug, B., Um, J., Baillargeon, J. N. and applications. Rep. Prog. Phys., Vol. 64, Number 11, Lin, C.-H. Grating-tuned external-cavity quantum- 2001, pp. 1533-1601. cascade semiconductor lasers. Appl. Phys. Lett., 25. Scarpa, G. Design and fabrication of Quantum Cas- Vol. 78, Issue 19, 2001, pp. 2834-2836. cade Lasers. PhD thesis, Technical University of Mu- 38. Totschnig, G., Winter, F., Pustogov, V., Faist, J. nich, 2002. and Mller, A. Mid-infrared external-cavity quantum- 26. Scamarcio, G., Capasso, F., Sirtori, C., Faist, J., cascade laser. Opt. Lett., Vol. 27, Issue 20, 2002, pp. Hutchinson, A. L., Sivco, D. L. and Cho, A. Y. High- 1788-1790. Power Infrared (8-Micrometer Wavelength) Superlat- 39. Peng, C., Luo, G. and Le, H. Q. Broadband, con- tice Lasers. Science, Vol. 276, Number 5313, 1997, tinuous, and ﬁne-tune properties of external-cavity pp. 773-776. thermoelectric-stabilized mid-infrared quantum- 27. Tredicucci, A., Gmachl, C., Capasso, F., Sivco, D. L., cascade lasers. Appl. Opt., Vol. 42, Issue 24, 2003, Hutchinson, A. L. and Cho, A. Y. Long wavelength pp. 4877-4882. superlattice quantum cascade lasers at ≈ 17µm. Appl. Phys. Lett., Vol. 74, Issue 5, 1999, pp. 638- 640.

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